Generally, in a mobile communication system, transmission and reception of signals are performed via radio links (or a downlink and an uplink) between a mobile station and a base station.
As shown in FIG. 1, a base station Node-B transmits a signal to a mobile station UE via a downlink (DL), and the mobile station UE transmits a signal to the base station Node-B via an uplink (UL).
Specifically, the base station Node-B and the mobile station UE use various channels set up on the downlink and the uplink, respectively, for signal transmission.
For the signal transmission via the downlink, the base station Node-B configured to use two antennas to divide transmission power into equal halves and transmit downlink signals (or DL signals) as shown in FIG. 2(b) can improve radio communication quality, as compared to that configured to simply use a single antenna to transmit a downlink signal (or a DL signal) as shown in FIG. 2(a).
Herein, the configuration shown in FIG. 2(b) is called a “transmission antenna diversity”, and the base station Node-B that implements the transmission antenna diversity is called a “transmission diversity station”.
The transmission antenna diversities fall into two broad categories: an “open loop transmission antenna diversity” shown in FIG. 3(a) and a “closed loop transmission antenna diversity” shown in FIG. 3(b).
For the purpose of improving the radio communication quality, there are also a plurality of systems in which the transmission diversity station (or the base station Node-B) applies different signal patterns, different weights of transmission power, and the like to two antennas, when transmitting downlink signals with the two antennas.
As employed herein, the “open loop transmission antenna diversity” shown in FIG. 3(a) is the system that does not require feedback information from the mobile station UE by applying predetermined signal patterns, predetermined weights of transmission power, and the like.
The “closed loop transmission antenna diversity” shown in FIG. 3(b) is the system that regularly receives optimum signal patterns, optimum weights of transmission power, and the like, as feedback information, from the mobile station UE according to the status of radio.
Generally, the “closed loop transmission antenna diversity” is applied to dedicated channels of each mobile station UE, because the application of the “closed loop transmission antenna diversity” can achieve an improvement in the radio communication quality, as compared to the application of the “open loop transmission antenna diversity”.
In contrast, the “open loop transmission antenna diversity” is applied to a common channel through which a plurality of mobile stations UE receive signals, because the common channel cannot be adapted to feedback information from a specific mobile station UE.
In the third generation mobile communication system “W-CDMA” system whose specifications are being standardized by the international organization for standardization “3GPP”, physical channels having specifications already standardized include a dedicated channel (DPDCH: Dedicated Physical Data Channel) that acts to transmit a data signal dedicated to each mobile station, and a dedicated channel (DPCCH: Dedicated Physical Control Channel) that acts to transmit a control signal dedicated to each mobile station associated with the DPDCH.
As shown in FIG. 4, communications between the mobile station UE and the base station Node-B are accomplished, by setting the DPDCH/DPCCH for the uplink and the downlink, bidirectionally.
According to specifications standardized by the 3GPP, the transmission antenna diversity can be implemented in the base station Node-B.
Specifically, the “STTD (Space Time block coding based Transmission antenna Diversity)” which is a type of open loop transmission antenna diversity, and the “CL TxDiv mode-1 (Closed Loop Transmission diversity mode-1)” or the “CL TxDiv mode-2” which is a type of open loop transmission antenna diversity, can be applied to the downlink DPDCH/DPCCH. The “CL TxDiv mode-1” and the “CL TxDiv mode-2” will hereinafter be collectively called “CL TxDiv”.
Generally, the “CL TxDiv” is applied to the downlink DPDCH/DPCCH, because the application of the “CL TxDiv” can achieve an improvement in the radio communication quality, as compared to the application of the “STTD”.    [Non-patent Document 1] “3GPP TS 25.211 v6.0.0,” December 2003
In addition, the 3GPP promotes the study of an “uplink high efficient transmission method (EUL: Enhanced Up Link)”, in order to enhance the efficiency of data signal transmission via the uplink.
As a result, there is a trend toward the additional provision of an “ACKCH (ACK CHannel)”, specifically, an “E-HICH (E-DCH HARQ Acknowledgement Indicator Channel)”, as a downlink control channel in conformity with the EUL. Incidentally, the E-HICH is a physical channel that acts to transmit layer-1 acknowledgement information dedicated to each mobile station UE.
There is also a trend toward the additional provision of an “E-RGCH (Enhanced Relative Grant CHannel)” and an “E-AGCH (Enhanced Absolute Grant CHannel)”, as downlink control channels in conformity with the EUL. Incidentally, the E-RGCH and E-AGCH are dedicated physical channels that act to transmit uplink rate assignment channel dedicated to each mobile station UE.
Although the transmission diversity station has to apply the transmission antenna diversity to all downlink channels, the 3GPP does not give specifications that define how the transmission antenna diversity should be applied to the E-HICH, the E-RGCH and the E-AGCH.